This proposal focuses on the critical epigenetic role of thyroid hormones in neuronal development. The tadpole nervous system offers several advantages for such studies, the major one being the coexistence of developing and mature neurons. This will allow us to investigate and compare mechanisms underlying the growth and differentiative responses to thyroid hormone in neurons targeted to the hindlimb, with mechanisms responsible for degenerative responses to these hormones in neurons targeted to the tail. We propose, first, to establish that lumbar dorsal root ganglia contain hindlimb-targeted neurons as well as tail-targeted neurons. Subpopulations of neurons will be identified by morphological characteristics and by staining after backfilling axons at their target tissues. The prediction that thyroid hormones stimulate protein synthesis in the developing hindlimb neurons and depress synthesis in the mature tail neurons will be examined by autoradiography. Second, we will confirm and extend preliminary findings that the thyroid hormone, triiodothyronine (T3), exerts differential effects on fast axonal transport of protein in the separate spinal nerve trunks from the lumbar ganglia to the hindlimb and the tail. Effects of T3 on fast transport will be compared at different periods of hindlimb growth and tail resorption. Our third aim is to develop a more complete model of endocrine effects occurring during metamorphosis. Since tadpole metamorphosis is accelerated or delayed by corticosteroids or prolactin, respectively, their abilities to modulate effects of T3 on protein synthesis will be examined in tissues that are programmed for development (hindlimb) or for degeneration (tail); subsequent studies will compare lumbar dorsal root ganglia (presumed to contain both mature and developing neurons) vs. tail dorsal root ganglia (containing mature neurons only). Our fourth aim is to investigate differential actions of T3 at the level of its nuclear thyroid hormone receptors. T3 binding characteristics will provide indications of the possible presence of multiple hormone receptors. Finally, since the physiologically most important class of T3 receptors are DNA-binding proteins, the number, size and sequence of DNA regions to which the receptors bind will be determined in T3 up-regulated and down-regulated tissues.